1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a method for manufacturing the same. Especially the present invention relates to a vertical-cavity surface-emitting semiconductor laser for long wavelengths (i.e., 1.3 to 1.55 .mu.m) to be used as an optical source for a system of optical communication, optical interconnection, optical data-processing, or the like, in the field of optical data-communication or optical data-processing, and also to a method for manufacturing the novel vertical-cavity surface-emitting semiconductor lasers for long wavelengths.
2. Description of the Related Art
Heretofore, the vertical-cavity surface-emitting lasers (VCSELs) have been investigated and developed as optical sources to be applied in the fields of optical-signal processing, optical-data processing, and so on because of their feasibility of two dimensional high-integration. That is, each of them comprises reflection layers consisting of semiconductor multi-layered films, respectively, and emits light at long wavelengths of 1 to 2 .mu.m that correspond to low-loss wavelengths of a quartz-crystal fiber. Hereinafter, such a kind of the VCSELs will be also simply referred as a long-wavelength VCSEL.
Referring now to FIG. 1, there is shown one of the examples of the conventional long-wavelength VCSELs as described in D. I Babic et al. Appl. Phys. Lett., vol. 66 (69), 1030-1032 (1995).
As shown in the figure, the VCSEL comprises: AlAs/GaAs reflection layers (mirrors) 1, 2 consisting of a plurality of AlAs layers and a plurality of GaAs layers, which are alternatively piled one by one into a stack; and an emission region consisting of InP cladding layers 3, 4 and an active layer 5 positioned between the cladding layers 3, 4. As shown in the figure, furthermore, the emission region is sandwiched between the mirrors 1, 2 and they are directly fused together. That is, the InP cladding layers 3, 4 on both sides of the active layer 5 are directly fused to the AlAs/GaAs mirrors 1, 2, respectively. This kind of direct wafer-fusion between the different semiconductors, such as the GaAs semiconductor and the InP semiconductor, is also disclosed in Japanese Patent Application Laying-open No. 7-335967 (1995).
The reasons for applying the step of direct wafer-fusion between the different semiconductors in the process of manufacturing the long-wavelength VCSEL as described above will be explained below.
A substantially small difference among refractive indexes of semiconductor layers (e.g., InP and InGaAs) of a reflection layer can be observed when the reflection layer is formed using the InP related semiconductor that emits light at a long wavelength, resulting in the difficulty of obtaining a sufficient degree of reflection. Therefore it is necessary to use another reflection layer, for example one made of AlGaAs/GaAs, with a large difference between the refractive indexes of its constituent layers. However it is difficult to grow the Ins semiconductor layer on a GaAs semiconductor (or vice versa). Even if the crystal growth is attained, a crystal defect such as a dislocation extends to a long distance from a growth interface into a growing film. In the case of the direct wafer-fusion, on the other hand, a crystal defect generated on the interface of the fused layers extends in parallel with the interface without entering into the fused layers, so that it reduces the tendency to introduce the crystal defect into the layers. Therefore, a person skilled in the art uses a structure prepared by forming an emission region using the InP semiconductor and fusing it with a highly-reflective GaAs reflection layer.
However, in spite of no observable macroscopic defect such as a dislocation in the active layer 5 of the above VCSEL, an emitting efficiency of the active layer 5 decreases as it were caused by a point defect. Therefore, the conventional VCSELs should be improved because of the following points (I) to (III).
(I) In the conventional surface-emitting laser described above, its laser characteristics including a threshold of current density for laser oscillation deteriorate as a result of causing a recombination center in the active layer which may be dependent on a point defect to be generated at the time of forming cladding layers 3, 4 as thin films so as to lessen the space between the active layer and the fused interface. For preventing the deterioration of the laser characteristics, on the other hand, there is an idea of forming the cladding layer 3, 4 thicker so as to open up the space between the fused interface and the active layer 5. In this case, however, there is a problem of increasing a threshold of current density for laser oscillation as a result of increasing the degree of optical loss in the cladding layers 3, 4.
(II) It is difficult to know an oscillation wavelength before direct wafer-fusing between different-typed substrates in the process of manufacturing the conventional VCSEL. Therefore, the person skilled in the art cannot adjust the observed oscillation wavelength to a predetermined one by repeating the steps of detecting the oscillation wavelength immediately after growing the emission region or the mirror layers 1, 2 and regulating the condition of growing the cladding layers 3, 4 according to the results of the detection. In the conventional VCSEL, accordingly, there is another problem of the difficulty in a precise regulation of the oscillation wavelength.
For widely distributed wavelengths of a gain spectrum in the active layer, an emission wavelength of the VCSEL is dependent on an optical phase shift per a reciprocating motion in a resonator with taking into account by the phase shift by the reflection layers 1, 2 as is the case with the distributed feedback waveguide laser. That is, an oscillation of the VCSEL can occur at a wavelength (i.e., a resonance mode) by which the sum of the following (1), (2), (3) is equal to an integral multiple of 2.pi.: (1), the phase difference between an input light and an output light of the reflection layer 1; (2) the phase difference between an input light and an output light of the reflection layer 2; and (3) a phase shift to be caused in a reciprocating motion of the light in the emission region which is sandwiched between the reflection layers 1, 2. Therefore, the oscillation wavelength of the conventional VCSEL can be determined after the step of wafer-fusion in which the reflection layers 1, 2 are fused with the emission region. It means that it is very difficult to know the oscillation wavelength of the conventional VCSEL before performing the wafer-fusion.
(III) For preparing a buried hetero-structure for simultaneously attaining current confinement into the active layer 5 and a lateral signal mode oscillation, it is required to etch to some midpoint of the InP cladding layer 4 under the active layer 5. However there is the difficulty of controlling a depth of the etch at constant. As a result, a structure of the buried layer is hardly formed to the predetermined design. In addition, a large leakage current occurs as a result of the improperly buried layer structure, and also the yields thereof decline.
Accordingly, the conventional VCSEL having the above problems is not a suitable light source of any one of the systems such as optical interconnection and optical data-processing systems that use a low threshold current (i.e., a low power consumption) and optical communication systems that require a precisely-controlled long wavelength. Even though there has been required a novel VCSEL to be driven at a low threshold current and to be precisely-controlled so as to have a predetermined emission wavelength in a long wavelength band as a communication wavelength band, the prior art has not arrived at a solution of the above problems because of many difficulties depending on the conventional device structure described above.